1. Field of the Invention
The present invention relates to a method of working a metal member, and more particularly, it relates to an improvement in a lubricating method while working a material consisting of a corrodible metal material such as a ferrous metal other than stainless steel, or a material consisting of a relatively work-hardenable metal material having high corrosion resistance such as stainless steel or a titanium alloy, and forming a prescribed member.
2. Description of the Background Art
Referring to FIGS. 1A to 5, conventional steps of working a ferrous material related to the present invention will now be described with reference to an example of steps of manufacturing a T nut including a step of intermittently progressively press working a plate of a ferrous metal material other than stainless steel, such as a carbon steel plate, for example, and a step of forming a female screw with a tapping machine.
A T nut 31 shown in FIGS. 1A and 1B is generally manufactured through a progressive press working step shown in FIGS. 2A and 2B. Referring to FIGS. 1A and 1B, the T nut 31 manufactured by this method comprises a shaft part 32 and a flange part 33 outwardly extending from a first end portion of the shaft part 32. The shaft part 32 comprises a female screw forming portion 35 which is in the form of a hollow cylinder having a uniform outer diameter and provided with a female screw 36 on its inner peripheral surface.
Two pairs of pawls 37, 38, 39 and 40 are arranged on the outer periphery of the flange part 33 to be opposite to each other along the diametrical direction of the flange part 33. Each of the pawls 37 to 40 is formed by bending upright a part of the outer peripheral edge of the flange part 33. The flange part 33 has a substantially octagonal shape as a whole. In particular, the pair of pawls 37 and 38 and the other pair of pawls 39 and 40 are connected with each other by linear sides 41 and 42 respectively.
The T nut 31 shown in FIGS. 1A and 1B is manufactured as follows. Referring to FIGS. 2A and 2B, first a longitudinal strip-shaped metal plate 10 consisting of a ferrous material is prepared. This metal plate 10 is caused to move intermittently pitch by pitch along its longitudinal direction as shown by arrows X in FIGS. 2A and 2B, to be subjected to desired working at each stopped position. Therefore, working zones i to vii in FIGS. 2A and 2B successively show working steps carried out in order in a single working space along the metal plate 10.
As shown on the right end of FIG. 2B, a lubricant is continuously supplied to the upper surface of the moving metal plate 10 from a supply port 50 of a lubricant supply unit. In the steps of manufacturing the T nut 31 from a ferrous plate, mineral oil, animal oil or vegetable oil having a rust preventive function is generally employed as the lubricant for the material itself which is easy to rust.
First, cut lines 11a and 11b are formed on the metal plate 10 by cutting substantially along a circular shape in the working zone i, for locating a portion for forming the flange part 33. At this time, uncut parts 12a and 12b are defined in two opposite portions of the circle defined by the cut lines 11a and 11b along the cross direction of the strip-shaped metal plate 10. Then, cut lines 13a and 13b are formed generally along a second circle that is concentric with and outside of the circle defined by the cut lines 11a and 11b, in the working zone ii. These cut lines 13a and 13b define uncut parts 14a and 14b in the vicinity of both edges along the longitudinal direction of the metal plate 10.
Then, a worked portion 16 enclosed within the inner cut lines 11a and 11b is drawn in the working zone iii. Drawing margins resulting from this drawing appear as clearances 15, and the worked portion 16 is held by coupling portions 17a, 17b, 17c and 17d connected with the uncut parts 12a, 12b, 14a and 14b with respect to the metal plate 10. The coupling portions 17a, 17b, 17c and 17d are adapted to advantageously absorb the drawing margins resulting from the drawing by deforming toward the central direction.
Then, the worked portion 16 is subjected to deeper drawing in the working zones iv and v, and thereafter a hole 18 is formed in the working zone vi. Thereafter, the hole 18 is spread as shown in the working zone vii.
Thereafter the metal plate 10 is passed through a step (not shown in FIGS. 2A and 2B) of forming the pawls 37 to 40 and the like, and then an intermediate product of the T nut 31 is separated from the metal plate 10 and barrel-polished. Then, the intermediate product is annealed in a vacuum furnace at a prescribed temperature, for relaxing work hardening that was caused during the progressive press working step, for facilitating tapping that will be carried out in a next step for forming the female screw 36, and for demagnetizing the intermediate product of the T nut 31 that became magnetized during the progressive press working step. The mineral oil or the like employed as the lubricant is inflammable and may ignite at the temperature necessary for the annealing. Therefore, it is necessary to carry out a step of cleaning the intermediate product with a solvent such as trichlene for removing the oil forming a coat on its surface before the annealing step.
The intermediate product of the T nut 31 annealed in the vacuum furnace is supplied to a tapping machine and subjected to a tapping step for forming the female screw 36 on the inner peripheral surface of the female screw forming portion 35 of the shaft part 32. Also in this tapping step, an oily lubricant such as mineral oil is employed similarly to the progressive press working step.
The tapping step for forming the female screw 36 on the inner peripheral surface of the female screw forming portion 35 of the intermediate product of the T nut 31 by the tapping machine will now be described with reference to FIGS. 4 and 5. A bent tap 71 which is held in a hollow rotary spindle 70 to rotate at about 1000 to 3000 rpm is employed in this tapping step. A plurality of T nuts 73 continuously supplied along a chute 72 are guided one by one to an end of the bent tap 71 through a stopper 74, and pressed against the bent tap 71 by a push rod 75, to be subjected to thread cutting. The bent tap 71 rotates along arrow Y with the rotary spindle 70, whereby each T nut 73 subjected to thread cutting moves along the bent tap 71 as shown by arrow Z, and is pressed by the subsequent T nut 73 to continuously move after passing through a cutting edge portion of the bent tap 71 held by tap covers 76 from both sides. Consequently, each T nut 73 passing through the inside of the rotary spindle 70 and reaching the other end of the bent tap 71 is discharged to the exterior from an opening 70a of the rotary spindle 70. Thus, the female screw 36 is completely formed, and the desired T nut 31 is obtained.
When the aforementioned T nut 31 worked from a plate of a ferrous material other than stainless steel is applied to a component member which is employed in a watery environment or an environment that readily corrodes metals, e.g. with acid rain, the color of the T nut 31 is disadvantageously changed by rust. Thus, there is an increasing demand for a T nut, consisting of a corrosion-resistant material such as stainless steel SUS304 under Japanese Industrial Standards (JIS), which is rustproof under such an environment.
The aforementioned conventional plate of a ferrous material other than stainless steel has relatively high heat conductivity and is hardly work-hardened during plastic working thereof. Therefore, no particular problem is caused during working steps such as drawing when a lubricant containing oil and water and having a strong cooling effect is employed. Further, such a ferrous plate is generally provided with a satin-finished surface having small convex and concave portions. During working, therefore, oil adheres to and remains in the concave portions to maintain excellent sliding between a tool and a workpiece, and serves as a kind of cushion for preventing extreme noise during the working.
However, it has been proved that, when a T nut is prepared from a plate of stainless steel while using a water-insoluble lubricant such as mineral oil similarly to the aforementioned conventional steps of working a ferrous plate into a T nut, the following problems are caused and excellent workability cannot be attained. This arises because stainless steel is relatively hard and inferior in plastic deformability, it has relatively low heat conductivity, it is readily work-hardened, and its surface is smoother than that of a carbon steel plate or the like and hence inferior in oil adhesion.
When the shaft part 32 of the T nut 31 is formed by drawing in the progressive press working step for the plate as described with reference to FIGS. 2A and 2B, the amount of heat generation is maximized in the vicinity of the bottom portion (the forward 15 end portion of the shaft part 32) where the maximum working stress arises. Since the heat conductivity of stainless steel is so low as compared with carbon steel or the like, there is insufficient time for the temperature in the plate to be uniformalized by heat conduction, leading to maximum work hardening in the vicinity of the bottom portion. When an oily lubricant such as mineral oil is employed in the progressive pressworking step, oil having a large cooling effect forms a coating on the surface of the workpiece, and hence the work-hardened portion is cooled in the hardened state. Once such work hardening is caused in a single stage of drawing in the multistage drawing operation shown in FIGS. 2A and 2B, then the desired drawing cannot be performed in the next stage due to the hardened condition and resulting reduced flexibility of the plate. Thus, the progressive press working steps cannot be smoothly performed.
Further, the surface of the stainless steel plate is smoother than that of the carbon steel plate, having smaller irregularity and hence inferior adhesion with an oily lubricant. Thus, lubricity between the tool and the workpiece is deteriorated in working, leading to extreme frictional noise during the working.
The reason why the stainless steel plate is so hard to press-work as compared with the carbon steel plate or the like will now be described with reference to FIGS. 3A to 3C. In the stages of drawing corresponding to the working zones iii to v shown in FIGS. 2A and 2B respectively, the metal plate 10 held between female dies 21a, 21b and 21c and blank holders 22a, 22b and 22c is drawn with punches 23a, 23b and 23c. If it consists of carbon steel or the like, the metal plate 10 maintains a substantially uniform thickness between a base portion and a bottom portion in circles shown by arrows A and B. If the metal plate 10 consists of stainless steel or the like, however, the flow of the material from the base portion to the bottom portion in the circles shown by arrows A and B (the flow in the section of the metal plate 10 along arrows in FIGS. 3A to 3C) is so minimal or inferior that the base portion may remain rather thick, the thicknesses of the side surfaces become non-uniform, and the bottom portion may become undesirably thin. This phenomenon is conceivably caused since no lubrication is maintained between the dies 21a, 21b and 21c and the punches 23a, 23b and 23c and the metal plate 10. Instead friction is increased due to inferior adhesion between the lubricant and the stainless steel plate surface, and the material is work-hardened by the frictional heat.
Such a tendency of ununiformalization of the thickness caused by drawing is accumulated through the plurality of stages of drawing, and hence the achieved thickness of the shaft part 32 the T nut 31 may be remarkably different from the desired thickness in the working zone shown in FIG. 3C, and the bottom portion of this shaft part 32 may be broken in an extreme case.
If the oily lubricant such as mineral oil is employed also in the step of forming the female screw 36 and thread cutting is performed by tapping at the same working speed as that for a conventional T nut prepared from a carbon steel plate or the like, then burning will result due to heat generated between the tap and the material for the T nut, which hinders excellent thread cutting.
It is inferred that the aforementioned problems in drawing and thread cutting of the stainless steel plate are caused since the stainless steel plate has inferior heat conductivity and is readily work-hardened, and since the surface of the stainless steel plate is so smooth as compared with the carbon steel plate or the like that the conventional oily lubricant is readily repelled from the steel plate surface during the working even if it had been adhering to the surface. Namely, the oily lubricant cannot remain on the surface of the workpiece for maintaining lubricity which is necessary for the working.
The inventor has performed an experiment of working a stainless steel T nut similarly to the above, using a lubricant prepared by diluting "Castrol Iloform PS158", which is a water-soluble plastic working oil by Castrol Industrial Co., Ltd. used for drawing a stainless steel sink, to 10% with water in accordance with the recommended specification. The oil must conceivably be diluted with water, for preventing ignition of the oil by heat generated during working. As a result of the working experiment with the water-soluble plastic working oil, work hardening was remarkably caused in both drawing and tapping steps, and excellent workability was not attained. It is inferred that an excess cooling effect arose due to functions of the oil and water contained in the lubricant, and hence it was impossible to solve the problem of work hardening similarly to the aforementioned case of employing a mineral oil as a lubricant.
When a trial experiment similar to the above was carried out using an undiluted solution of the water-soluble plastic working oil, excellent workability was not attained similarly to the case of employing the 10% diluted solution. This is conceivably because an unpreferrable cooling effect arose due to the considerable amount of water that is contained in the undiluted solution of this water-soluble plastic working oil for preventing ignition of the oil contained therein. Hence the problem of hard working was not solved.
When an experiment for working a stainless steel T nut was performed similarly to the above using "High-Chip EX-446H", which is water-soluble lubricating oil by Taiyu Co., Ltd. containing components shown in Table 1, work hardening was remarkably caused and excellent workability was not attained. This is presumably because the lubricating oil contains mineral oil in a relatively large amount of 10 to 20%, and further contains large amounts of components such as a rust preventive additive and an antifoaming agent which are unnecessary for serving as a lubricant for stainless steel. This was the case even though the lubricating oil is mainly composed of a surface-active agent. Hence it was impossible to solve the problem of work hardening due to a cooling effect caused by these unnecessary components, similarly to the aforementioned case of employing mineral oil.
TABLE 1 ______________________________________ Composition of High-Chip EX-446H (Water-Soluble Cutting Oil) Component Content ______________________________________ surface-active agent 40-60% purified mineral oil 10-20% chlorine extreme pressure additive 10-20% sulfur extreme pressure additive 20-30% rust preventive additive 1-10% antifoaming agent not more than 1% ______________________________________
The problem of inferior working properties caused by work hardening resulting from inferior heat conductivity arises not only in stainless steel, but is common to materials such as a titanium alloy having high corrosion resistance. The development of a lubricating method which can solve this problem has been long awaited.
In case of machining a plate of a ferrous material other than stainless steel, which can attain relatively excellent workability with an oily lubricant such as mineral oil in general, the lubricating oil typically spills on the floor. Over time the lubricating oil deeply infiltrates into the ground and thereby contaminates ground water and other ground resources, to cause environmental pollution. Also in relation to working a corrodible, metal material, therefore, the development of an oil-free lubricant having excellent lubricity is much awaited.